The invention provides an isocyanate prepolymer composition useful in the preparation of flexible polyurethane foams which are particularly suitable in seating applications. The invention further provides a method of preparing the isocyanate prepolymer composition, a method of using the composition in the manufacture of polyurethane foams and the resultant foams. More particularly, the invention relates to an isocyanate prepolymer composition which results from combining a particular isocyanate blend, a particular polyhydroxy-containing polyol and a toluene diisocyanate.
The reactions of polyisocyanates with compounds containing at least two isocyanate-reactive hydrogen atoms are well known. When conducted in the presence of a catalyst and blowing agent, such reactions are used to manufacture both rigid and flexible polyurethane foams. Flexible polyurethane foams are distinct from rigid foams and have a limited resistance to an applied load, are open-celled and permeable to air and are reversibly deformable. See G. Hauptman, H.-A. Freitog and A. Volland, Polyurethane Handbook, Chapter 5, Sections 5.0 and 5.3-5.3.3 (Ed. G. Oertal 1985),hereby incorporated by reference.
Flexible foams are generally produced via a continuous slabstock process or a discontinuous molding process. Molding processes are advantageous because articles may be produced fairly easily and without waste. Flexible polyurethane molded foams are particularly suitable for use in seating applications in the home, office and automobile markets.
However, the production of flexible polyurethane molded foams having optimum performance properties can be challenging, particularly where the mold is relatively large and possessed of a complicated shape or configuration. In particular, if the viscosity of the foaming mixture increases to a certain point before the mixture reaches all portions of the mold, the mold will never be fully filled. Generally, the foam formulation will have greatly decreased mobility at it's string gel time or gelation stage. Foam formulations which gel before the mold is adequately filled are said to have inadequate flow.
When the foam formulation has inadequate flow, the resultant molded foams are of suspect quality. They are often prone to instability and collapse and exhibit impaired performance characteristics. Inadequate flowability characteristics can also be manifest as voids and other physical imperfections.
Several prior art attempts to address this problem have focused on the isocyanate component. Particular attempts have used combinations of toluene diisocyanate (TDI), and diphenylmethane diisocyanate (MDI) and/or (polymeric-MDI or polyphenylpolymethylene polyisocyanates PMDI). For example, U.S. Pat. No. 3,492,251 disclosed a mixture of polyisocyanates having 50 to 65 percent PMDI and 30 to 50 percent TDI. However, the foams produced were rigid foams lacking the performance characteristics desired in automotive seating.
U.S. Pat. No. 4,256,849 disclosed a process for the production of cold setting, flexible foams containing urethane groups wherein the isocyanate component was a mixture of diphenylmethane diisocyanates and polyphenyl-polymethylene polyisocyanates, both substantially free of carbodiimide groups, the mixture having an isocyanate content of greater than 30 percent. The mixture contained from 60 to 90 percent by weight of 4,4'-diphenylmethane diisocyanate, 3 to 30 percent by weight of 2,4'-diphenylmethane diisocyanate, and up to 20 percent by weight of other polyisocyanates including toluene diisocyanate isomers.
In U.S. Pat. No. 4,803,229 a molded flexible polyurethane foam was prepared by reacting a polyol with a urea-biuret modified polyisocyanate mixture composed of toluene diisocyanate and polymeric diphenylmethane diisocyanate. The disclosure required that either or both of the TDI and PMDI be modified with urea-biuret structures. The patent was directed toward obtaining molded flexible polyurethane foams with improved air flow and compression set.
Finally, U.S. Pat. No. 5,132,334 disclosed a polyisocyanate mixture having 21 to 95 percent by weight of toluene diisocyanate and 5 to 79 percent by weight of a mixture of diphenylmethane diisocyanates and polyphenylpolymethylene polyisocyanates containing (1) 55 to 87 percent by weight of 4,4'-diphenylmethane diisocyanate, (2) 8 to 30 percent by weight of 2,2'-diphenylmethane diisocyanate, (3) 0 to 4 percent by weight of 2,2'-diphenylmethane diisocyanate, and (4) 5 to 35 percent by weight of polyphenylpolymethylene polyisocyanates.
However, the prior art has failed to provide an adequate solution to the above problem. Thus, it is an object of this invention to provide an isocyanate component which has flowability characteristics such that large molds having complicated configurations may be adequately filled to produce flexible polyurethane molded foams having an optimum balance of performance properties.
More particularly, it is an object of the instant invention to provide an isocyanate prepolymer composition, capable of functioning as the isocyanate component in a flexible polyurethane foam formulation which exhibits improved flowability characteristics.
It is a further object of the invention to provide an isocyanate prepolymer composition which satisfies the latter object while simultaneously providing flexible polyurethane molded foams having improved ILD, compression set and flammability characteristics.